The reaction between an oxazoline group and a carboxyl group is known in public and used to make a crosslinked polymer. There has been disclosed in U.S. Pat. No. 4,474,923 a self-curable latex composition consisting of a latex containing oxazoline groups and a latex containing carboxyl groups. This latex composition cures by a reaction of the oxazoline group with the carboxyl group.
On the other hand, there are known catalysts for producing a crosslinked resin from a bisoxazoline compound and a dicarboxylic acid. U.S. Pat. No. 4,474,942 has disclosed a process to make a crosslinked polymer by heating a mixture of the bisoxazoline compound and dicarboxylic acid in the presence of a catalyst such as esters of organic phosphorous acids, inorganic salts. The inorganic salts are halides, nitrates, sulfates, or chlorates of the metal salts of from monovalence to tetravalence. U.S. Pat. No. 4,579,937 has disclosed a process to make a crosslinked polymer by heating a mixture of the bisoxazoline compound and dicarboxylic acid in the presence of oxazoline ring-opening catalysts. The oxazoline ring-opening catalysts are strong acids, sulfonic acid esters, sulfuric acid esters, or organic halides. Japanese Official Patent Provisional Publication, showa 60-161427, has disclosed a process to obtain highly polymerized polyesters by a reaction of a saturated polyester containing a terminal carboxyl group with a bisoxazoline compound under a melted condition in the presence of quarternary ammonium salts.
The aforementioned latex composition is useful in the fields of adhesives, paints, coatings, printing ink and so forth and its suitability for practical use has been required. The curing reaction of the latex composition, especially, the curing reaction at low temperature is very slow to proceed. The curing reaction at low temperature is generally accelerated by a catalyst.
However, the aforementioned catalysts can not accelerate the reaction between the oxazoline group and the carboxyl group, especially the reaction at low temperature and, in certain cases, they cause coagulation the aqueous resin composition.
It is an object of the present invention to provide an aqueous resin composition which does not cause coagulation and can be cured at low temperature and, by this curing, forms a coating superior in mechanical strength, water resistance, solvent resistance, and adhesion to various substrates.
Furthermore, it is an object of the present invention to provide a curing process which can form a coating superior in mechanical strength, water resistance, solvent resistance, and adhesion to various substrates without causing coagulation of a reacting substance and at low temperature.